Files disappear from file server

Hi there,

We have a small network of 5 pc's (xp sp3) and a server (2003 sp2 r2) at a remote office. The server acts as a file/print server and replicates files to an SBS server (2003 sp2 r2) at the main office. Very occassionaly we have an issue with files disappearing from the servers. The most recent example was a .xls file that one of the users had been working on all day, saving regularly. They went to a meeting and when they came back the file had gone. There is nothing in the error logs to suggest what might be responsible, and the user is absolutely sure that they did not accidentaly delete the file. Can anyone point me in the right direction?